In recent years, the signal frequency region for information and communication apparatus (e.g., PHS and mobile phones) and the CPU clock time of computers reach the GHz region and employment of a higher frequency has been in progress. The transmission loss of an electric signal is expressed as the sum of dielectric loss, conductor loss and radiation loss, and the dielectric loss, conductor loss and radiation loss increase with an increase of the frequency of the electric signal. Since the transmission loss decays the electric signal to deteriorate the reliability of the electric signal, a wiring board for high-frequency signals should be designed to suppress the increase of the dielectric loss, conductor loss and radiation loss. The dielectric loss is proportional to the product of the square root of the relative permittivity of an insulator forming a circuit, the dielectric loss tangent of the insulator and the frequency of signals used. Therefore, the increase of the dielectric loss can be suppressed by choosing an insulating material having a low permittivity and a low dielectric loss tangent, as the insulator.
Typical materials having a low permittivity and a low dielectric loss tangent are described below. Fluororesins represented by polytetrafluoroethylenes (PTFE) have both a low permittivity and a low dielectric loss tangent and hence have been used as a material for a substrate suitable for high-frequency signals since early times. On the other hand, various insulating materials containing no fluorine and having a low permittivity and a low dielectric loss tangent have been investigated which are easy to make into a varnish by the use of an organic solvent, have a low molding temperature and a low curing temperature, and are easy to handle.
JP-A-2002-249531, JP-A-2003-12710 and JP-A-2003-105036 disclose examples of employment of a polyfunctional styrene compound as a crosslinking component. In addition, in many of these examples, permittivity adjustment, flame retardation and strength improvement by the addition of an inorganic filler are described.
It is generally known that the conductor loss and the radiation loss can be reduced by forming a conductor wiring with a low surface roughness by the use of a metal having a low conductor resistance, such as gold, silver, copper or the like.
The cured products of resin compositions and prepregs disclosed in JP-A-2002-249531, JP-A-2003-12710 and JP-A-2003-105036 are materials having a low permittivity but have a dielectric loss tangent of more than 0.002. Thus, they are still unsatisfactory as an insulating material in their recent application at a high frequency. The present inventor investigated a method for further reducing the dielectric loss tangent of the cured products of resin compositions and prepregs, and consequently found that the dielectric loss tangent (dielectric loss) of the cured products could be reduced by adding a specific treating agent to the resin compositions or previously treating an inorganic filler, a base material (e.g., glass cloth), copper foil or the like.
Accordingly, an object of the present invention is to reduce the dielectric loss tangent of a cured product of a resin composition containing a crosslinking component having a plurality of styrene groups, a high-molecular weight compound and an inorganic filler.
Another object of the present invention is to provide a resin composition having both an excellent flame retardancy like that of the above-mentioned resin composition and a very low dielectric loss tangent and further provide a printed wiring board, a laminate sheet, a prepreg and a resin-layer-provided conductor foil which are obtained by using the resin composition.